1. Field of the Invention
This invention relates to a method for preparing a conductive silicone rubber composition, and more particularly, to a method for preparing a conductive silicone rubber composition which yields silicone rubber having stable conductivity in the semiconductive region and thus finding use as roll members in copying and facsimile machines.
2. Prior Art
Most rubbery materials are electric insulators. A number of conductive rubbers are available as mixtures of rubbery material and conductive agents. For example, rubbers having carbon black blended therein to provide an electric resistivity of from 10.sup.-1 to 10.sup.2 .OMEGA.-cm are conductive rubbers which are used in various applications.
Silicone rubber is also widely used as electrically insulating rubber because of its heat resistance, low-temperature resistance and weatherability. It can also be used as conductive silicone rubber by adding conductive agents as with other rubbery materials.
The conductive agents which are added to silicone rubber for imparting electric conductivity are typically carbon black, graphite, various metal powders such as silver, nickel, and copper, various non-conductive particles and short fibers surface treated with silver or similar metals,carbon fibers, and metallic fibers. By mixing these conductive agents, the resistivity of silicone rubber can be reduced to the order of 10.sup.10 to 10.sup.-3 .OMEGA.-cm depending on the type and amount of conductive agent without detracting from the inherent properties of silicone rubber.
Where silicone rubber is blended with conductive carbon black such as Ketjen Black and acetylene black so as to provide a resistivity in the semiconductive region of 10.sup.3 to 10.sup.10 .OMEGA.-cm, the resistivity experiences a large variation and is difficult to stabilize. This is probably because the dispersion of carbon markedly varies depending on molding conditions.
In the manufacture of business machine parts including those used in dry copying machines, for example, rubber rolls such as charging, transfer, developing and pressing rolls and rubber members such as cleaning blades and fixing belts, the industry is now searching for materials having semiconductive characteristics. Therefore, there is a need for rubber articles having minimized variation of electrical resistivity in the semiconductive region, that is, stable electrical resistivity.